Last week, a neighbour kissed my baby on his lips. I was so surprised, I didn't even respond. I think it's unhygienic and disgusting. Am I overreacting? How can I be polite, but make sure this doesn't happen again?

This is no overreaction, in fact this is the exact way to react! It's certainly unhygienic and unhealthy and in no way should be encouraged. The best way to deal with this problem is to bluntly tell this person to never repeat his/her actions again. After all it is the question of your child's safety. Be open and politely first up, don't allow physical contact with others. Tell them your child is susceptible to infections and you are avoiding him to have physical contact with outsiders. If you are bold enough, you are bluntly tell him/her to not kiss your child on the lips.